Several different designs of such circuit boards are known to the art.
The latest development within electronic equipment tends towards increasing signal speed and towards increasing the compactness of discrete circuits on circuit boards. These measures has led to problems relating to electromagnetic interference (EMI).
Various measures have also been proposed in endeavours to reduce or fully eliminate the electromagnetic interferences that result primarily from higher signal rates and short switch-over times.
Concerns other than EMI are related to electromagnetic compatibility (EMC) and electromagnetic discharge (EMD).
It is also known to fully "cover" or enclose circuits of this kind in an electrically conductive material so as to obtain effective screening to and from the surroundings.
It is also known to fully encapsulate circuit boards in a metallic casing and to provide delimited holes in the casing through which cables and conductors can be connected to the circuit board.
In the case of circuit boards, it is known to attempt to "cover" and enclose the signal-carrying conductors formed in one or more electrically conductive layers in the form of copper foils inwardly of a circuit board having external earth-potential related electrically conductive layers.
An example of this technique is given in publication EP-A2-0 238 267.
Such measures are taken in order to prevent external electromagnetic interference fields from obtaining access to the interior of the circuit board and also to prevent the emission of electromagnetic fields generated by board components as interference fields.
It is also known in circuit boards to endeavour to utilize mutually opposing external electrically conductive layers and to connect said layers to earth potential such as to give said layers a screening function.
However, such circuit boards normally require electric conductors to be included in these screening layers.
The provision of such conductors requires conductor-related foil sections to be isolated from the electrically conductive layer or from the remainder of the foil, wherein it is known to form said conductors by etching away selected sections of the metal foil, such as to enable the conductors to be formed without contact with the remainder of the metal foil.
In this regard, it is known to permit that part or those parts of the metal foil or the outer copper layers that have not been used as conductors for connection purposes and which have not been etched away to remain on the circuit board and to connect this part or these parts to earth, so that the layer will obtain a partial screening function.
It is quite natural that screening layers etched in this manner are unable to provide a complete or a maximum screen, since several different conductor patterns have been formed therein with peripheral exposure of the metal foil.
Even though encapsulation forms part of the earlier standpoint of techniques in preventing electromagnetic interference, this technique will not be described for the sake of simplicity.
The earlier standpoint of techniques also includes the realization that screening problems are heightened with increasing frequencies of electromagnetic fields and interference fields.
It has been discovered in the development of electronics towards still higher working frequencies and shorter switch-over times that a screening technique of the aforesaid kind is no longer satisfactory.
One of the reasons for this is apparently because the conductors formed in the outer metal layer form radiation sources. Foil sections that have been etched away provide screen-free surfaces through which fields and interference fields are able to pass.
In certain cases, these circumstances may result in a screening layer amplifying outwardly directed radiation and become highly receptive to external radiation or interference fields.
The reason for this is believed to be because chosen dimensions of openings provided in the copper layer for forming said conductors within the electrically conductive layer and the chosen lengths of the conductors can result in correspondence to an adapted antenna length or a part thereof for certain frequencies of functions used in the circuit board.
By way of example, it can be mentioned that an oscillation of 3 GHz in air results in a wavelength of 10 cm, and a conductor length of 2.5 cm. generates an antenna function having a 1/4 wavelength.
It will be noted that a conductor structure having the aforesaid adapted length is reciprocal, meaning that it functions both in a transmitting and a receiving mode, which in the latter case lowers the immunity that a screen is intended to offer, and certain length dimensions will afford good reception conditions for corresponding external interference frequencies.
With regard to a further standpoint of techniques in this regard, reference can be made to the disclosures made in U.S. Pat. No. 5,341,274.
A bottom layer (4) is comprised of a layer produced from insulating material and can be readily formed with the aid of a stencil-printing process. Subsequent to having formed the bottom layer (4), a conductive layer (5) is formed in a stencil-printing process within a region (12) located beneath an IC circuit. There is then applied a top layer (6) in the form of an insulating material formed around the whole surface of a substrate (1), with the exception of parts (2a) that are intended to receive the pin connectors of an integrated circuit.
The upper layer (6) can be formed from the same material as the bottom layer (4). The publication also discloses that the electrically conductive layer (5) is formed so as to have no contact with the connections (2a) of the integrated circuit in the region between the pin connections, as shown in FIG. 8b.
The frequency components generated within this region will thereby be limited and radiation noise can be dampened.
U.S. Pat. No. 5,274,193 also teaches a multi-layer circuit board which utilizes an electromagnetic screening layer produced from a treated copper paste and applied so as to cover substantially the whole surface.
Publication EP-A1-0 578 888 discloses that a circuit board may be provided with an electrically conductive rail which extends along mutually opposite sides and along a part of its rear side, therewith creating the possibility of connecting the circuit board to an earth plane and therewith create conditions for screening against EMI radiation.
The subject matter of U.S. Pat. Nos. 5,323,299, 5,335,147 and 5,006,667 also forms part of the prior art.